The present invention relates to intermittently illuminated warning lamps, and more specifically, intermittently illuminated warning lamps which employ gas discharge tubes as the source of illumination.
In the past, warning lamps, such as those commonly used as road side markers and those found on emergency vehicles, have generally employed sealed beam incandescent lamps as the source of illumination. In a standard design, two such lamps are mounted on a movable carriage and rotated at a rate of 30 revolutions per minute to produce flashes at the rate of 1 per second. The duration of individual flashes and the number of flashes per unit time can be increased by increasing the number of lamps used in the housing and reducing the rate of rotation of the lamps.
Recently, strobe-type lights, i.e. gas discharge tubes, have been employed in the flashing warning light environment. These strobe-type lamps provide a number of significant advantages over the incandescent lamp warning light systems described previously. Among these advantages are high flash intensity, extremely long lamp life, completely solid-state circuitry and components for control of the lamp, elimination of moving parts such as electric motors, gear trains and commutator brushes, flexibility in the designed range of voltages and currents used in the operation of the lamps, rugged shock and vibration resistance, low power input requirements and minimal maintanence due to a long lamp life, solid-state circuitry, elimination of moving parts and general ruggedness.
One disadvantage in the use of strobe-type lamps in warning light systems is the extremely short duration of the flash. This disadvantage may not be significant in periods of darkness or low ambient light levels, but it does present a serious problem during periods of high ambient light, particularly bright sunlight. The problem results from the inability of the human eye and brain to acknowledge a flash of high intensity and short duration. In bright sunlight, the eye pupils are closed down to accept a minimum amount of light. The normal strobe flash, even though of high intensity, is of such short duration that under bright ambient conditions the human brain will not respond to it adequately for the flash to properly serve as a warning light. At night, when the pupils of eyes are expanded to accept a maximum amount of light, the high intensity of the strobe flash at short distances can be somewhat blinding and confusing.
As a comparison, the strobe flash may have an intensity of over 1 million candle power but a very short duration, for example, only 1/30,000th of a second, while the sealed-beam incandescent lamp normally has an intensity of about 35,000 candle power but a duration of 1/12 of a second or more depending on the speed of rotation of the lamp. Thus, at night the incandescent lamp is less likely to produce an irritating flash. In bright sun light, even though it has a much lower candle power, the longer duration or dwell period of the incandescent lamp gives it greater visual acceptance to the human eye and brain under some ambient light conditions, and therefore greater value as a warning light in daylight conditions. This shortcoming of the strobe light to some degree can be overcome by increasing the length of the flash by increasing the size of the discharge capacitor and power supply, for example. However, the increased size and increased component cost makes this approach impractical.
It will be appreciated from the foregoing that in daylight and particularly bright sunlight the strobe flash is inadequate to draw attention to a traveling vehicle at distances greater than about 250 feet. An approaching motorist at 55 miles per hour would see only three strobe-light flashes while traveling 240 feet with a flash rate of 1 flash per second. The motorist could be in a collision path before being able to mentally and physically react to the approaching danger. With conventional electronic circuity, the flash rate can be easily increased to one and one half flashes per second. This would only mean that the motorist would see four or five flashes in the 240 feet instead of three. However, this distance is still too short for the motorist to react safely.
One attempt to increase the duration of a flash produced by a strobe-lamp is disclosed in U.S. Pat. No. 3,973,168 to Kearsley. This patent discloses a warning circuit in which three strobe-lamps are arranged to be sequentially flashed. The sequential flashing of the strobe-lamps can be close enough to give the visual impression of a single flash of long duration. The disclosed apparatus requires three separate power supplies and three separate triggering units, one for each strobe lamp. The cost of all the required components does not provide an economical means for utilizing a strobe lamp as a flashing indicator.
An approach similar to that of the Kearsley patent has been attempted utilizing two strobe lamps. The lamps were alternately flashed so as to approximately appear as a single flash of long duration. Again, however, the cost of such a device is not practical.
Another known device that increases the duration of a flashing strobe lamp is disclosed in U.S. Pat. No. 3,430,102 to Sidur. This patent discloses a circuit for causing a gas discharge illumination device to emit two flashes over a short duration of time. The two flashes can be closely spaced to give the effect of a single flash of long duration. However, the patent only teaches the production of two closely spaced flashes, and therefore, the duration of the single visible flash is limited to that which can be produced by two closely spaced flashes.
U.S. Pat. No. 4,013,921 to Corthell also discloses a circuit for causing a gas discharge lamp to produce two closely spaced flashes. In the circuit disclosed by this patent, the second flash is of substantially lesser intensity than the first flash. Consequently, the Corthell device not only suffers from the disadvantage of other prior art, multi-flash systems, but also presents a problem with overall intensity since the intensity of the "effective" single visible flash falls off rapidly.